Colloidal particles such as nanospheres, liposomes, and micelles have been studied extensively for site-specific drug delivery. Generally, such particles must escape capture by the reticuloendothelial system (RES) of the liver and the great filtration activity of the lungs if they are to deliver drugs to other tissues. In recent years, survival of colloidal systems in the blood has been improved by the use of PEG-containing amhiphiles (Lasic et al., Ed. Stealth Liposomes; CRC Press: Boca Raton, Fla., 1995). As a result of the PEG, macrophage clearance of PEG-based liposomes has been drastically reduced by decreasing opsonization by plasma proteins (Torchilin et al., Biochim. Biophys. Acta 1994, 1195, 11–20). Furthermore, a variety of ligands, such as antibodies, growth factors, and cytokines, have served to enhance the delivery capabilities of PEG-coated liposomes, and it has been demonstrated that the maximal activity is shown by ligands tethered to the distal end of PEG chains (Blume et al., Biochim. Biophys. Acta 1993, 1149, 180–184; Zalipsky et al., Bioconjugate Chem. 1995, 6, 705–708; Zalipsky, J. Controlled Release 1996, 39, 153–161; Gabizon, Bioconjugate Chem. 1999, 10, 289–298). Some of these ligands can lead to very efficient cellular uptake, such as the use of growth factors, for example, fibroblast growth factor to effect cellular uptake of DNA formulations.